The invention relates to a paper sensor, and more particularly, to a light weight paper sensor.
In machines requiring the movement of sheets of paper in timed sequence, such as printer and reproduction machines, paper jamming often occurs due to improper paper feeding, spacing inaccuracies, and various other factors resulting in the improper deceleration or acceleration of paper sheet speeds in the machine.
The prior art is replete with paper sensing devices to sense the presence of sheets of paper at various points along the path of travel. One type of sensing device takes the form of switches actuated by switch arms located in the path of movement of the sheet. The disadvantages of this type of sensor are the response time of the control to the mechanical actuation of the switch by the paper. Also, the fact that the sheet of paper must contact a switch arm itself may effect the travel of the sheet, either retarding the advancement of the sheet or skewing the sheet out of the predetermined path.
Other types of detection devices teach the use of a plurality of ultrasonic detecting devices dispersed along the path. Each of the detecting devices includes an ultrasonic transmitting transducer for generating ultrasonic waves of a predetermined wavelength and an ultrasonic receiving transducer to receive the transmitted waves. The acoustically vibrating element in each of the transducers is generally a piezoelectric material for converting electrical signals to mechanical vibrations or mechanical vibrations to elecrical signals. The detection devices are arranged along the paper path and circuit means monitor in timed sequence the effect upon each of the detection devices as sheets are transported.
Other sensors teach the use of a hollow tube attached to a speaker. In operation, the speaker is driven at constant frequency and the acoustical impedance of the hollow tube is measured by measuring the electrical impedance of the speaker. When a document moves along the paper path and covers one end of the hollow tube, the acoustical impedance and thus the electrical impedance is changed. A difficulty with this type of system and other prior art systems is that it is necessary to provide a separate transmitter and/or receiver along each port or location of the paper path where it is desired to sense the presence or absence of a sheet of paper. This can be relatively complicated and expensive.
One of the problems with most of the paper path sensors that are used today is the inability to respond well to papers of light weight. Because of the thinness of the paper, it is very easy for the light from an optical sensor to go through the paper and be received by a detector. With a mechanical switch it is easy for the paper to be deflected and not trip the switch because the paper is not very stiff. One attempt to solve this problem is described in IBM Technical Disclosure Bulletin, vol. 22 No. 4, September 1979 showing an LED light source beam twice broken by the same document before the beam is detected by a phototransistor. However, because of the mirror or prism reflecting surfaces, it is relatively more difficult to focus the light beam on the paper path target area. Also, the direct impact of the beam on the paper, even though striking the paper twice, generally contributes to the penetration of the light beam through the paper after resulting in an improper detection or paper present signal.
It would be desirable, therefore, to provide a sensor system that overcomes these difficulties in the prior art, and in particular, minimizes the potential for error signals in sheet detection and focuses the light beam on the paper path target area, to be able to detect relatively thin sheets. It is an object of the present invention, therefore, to provide a new and improved optical, thin sheet detector that sufficiently blocks the light path to the detector to signal the presence of a sheet in the paper path. It is another object of the present invention to provide an angled path of the light beam to the paper or sheet and to provide an optical fiber in the light path to focus the light onto the target area. Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.